1. Field of the Invention
The present invention relates to an ohmic electrode structure provided in a compound semiconductor device, a semiconductor device including such an ohmic electrode structure; and a method for producing such a semiconductor device.
2. Description of the Related Art
In a conventionally known method for providing ohmic contact with an n-GaAs layer in a compound semiconductor device, an In.sub.x Ga.sub.1-x As layer (0&lt;x.ltoreq.1) is epitaxially grown on an n-GaAs layer, and then a metal or alloy layer is formed thereon.
FIGS. 9 and 10 each show a conventional ohmic electrode structure.
In the structure shown in FIG. 9, an In.sub.x Ga.sub.1-x As layer 2 is provided on an n-GaAs substrate 1, and a Ti layer 41, a Pt layer 42 and a Au layer 43 are provided on the In.sub.x Ga.sub.1-x As layer 2 in this order. The Ti layer 41, the Pt layer 42 and the Au layer 43 form an electrode.
In such a structure, since the three metal layers 41 through 43 forming the electrode easily react with the In.sub.x Ga.sub.1-x As layer 2 which is lattice-mismatched, the heating temperature cannot be very high after the electrode is formed. For example, when the structure is heated at 390.degree. C. for 1 minute, the contact resistance .rho.c increases by about three orders, from 1.times.10.sup.-8 .OMEGA.cm.sup.2 to 1.times.10.sup.-5 .OMEGA.cm.sup.2.
In the structure shown in FIG. 10, which is disclosed in Japanese Laid-Open Patent Publication No. 1-194468, a barrier layer 5 formed of tungsten silicide (WSi) is interposed between the In.sub.x Ga.sub.1-x As layer 2 and the Ti layer 41.
In such a structure, the contact resistance is stable with no significant increase even if the structure is heated at 400.degree. C. or more due to the presence of the barrier layer 5. However, in the case when the barrier layer 5 is formed by sputtering, it is difficult to obtain a desirable composition for the barrier layer 5 because the sputtering efficiency of tungsten is different from that of silicon. Further, the purity of WSi, which is a sintered body, cannot be very high. Moreover, WSi is generally present in a columnar polycrystalline structure, and thus is difficult to process, which complicates the formation of the ohmic electrode structure.
In a conventional semiconductor device such as a heterojunction bipolar transistor (HBT) including semiconductor layers respectively having p-type and n-type conductivities on one substrate, different electrodes need to be formed respectively for the semiconductor layers. This is because materials which can be used for different electrodes to be in ohmic contact with the p-type and n-type semiconductor layers are different from each other. Thus, the fabrication process of the semiconductor device is complicated, and the electrodes easily become non-uniform in thickness and characteristics.